1. Field of the Invention
The present invention relates to a repair method for a memory device that repairs failed cells of a memory device by using redundancy of spare rows or spare columns.
2. Description of the Related Art
A fault repair method of a memory device according to the prior art will be explained below with reference to FIGS. 1-4.
First, in Steps 11 and 12, it determined whether or not the device can be repaired based on information on the number of failed cells produced through device measurement. As shown in FIG. 2, the number of lines of spare columns and spare rows is Sc and Sr, respectively; and if the number of cells per row that can be repaired is r and the number of cells per column that can be repaired is c, then the maximum number of failed cells that can be repaired is (Sr.times.c)+(Sc.times.r)-(Sr.times.Sc). If the number of failed cells exceeds this maximum number of repairable failed cells, repair cannot be carried out.
If repair is possible, repair of failed lines is performed in Step 13. As shown in FIG. 3, the number of lines in a spare column and in a spare row is Sc and Sr, respectively, and the number of failed cells at row address r as seen from the column direction is Fr. If Fr&gt;Sc, repair of all of the failed cells at row address r is not possible unless spare rows are used, and accordingly, row address r becomes one repair solution. This check is continued for row addresses and column addresses.
If repair is determined to be possible in Step 13, repair solutions are found, in Step 15, for the remaining failed cells by a round robin approach. Focusing on the repair of one failed cell, if both a spare row and a spare column exist, there are two repair solutions for the failed cell. In dealing with the next failed cell, if both a spare row and spare column exist, the two repair solutions for the preceding failed cell each have a further two repair solutions, for a total of four repair solutions. This approach is applied to all failed cells, and all possibilities are investigated to find the optimum repair solution having the smallest number of spare columns or spare rows. If a repair solution which is capable of repairing all of the failed cells cannot be found, then thr repair is impossible.
FIG. 4 shows repair solutions for a case wherein both the number of spare rows and the number of spare columns are three, and there exist eight failed cells. First looking at failed cell a, there are repair solutions in which a row address of failed cell a is repaired, and a repair solution in which a column address of failed cell a is repaired. Repair solution 1, Repair solution 2, Repair solution 3, and Repair solution 5 are solutions in which a row address of failed cell a is repaired, and Repair solution 4 is a solution in which a column address of failed cell a is repaired.
A repair solution in which a row address of failed cell a is repaired inevitably repairs failed cells b and c. Directing attention now to failed cell d, there are repair solutions in which a row address of failed cell d is repaired and a repair solution in which a column address of failed cell d is repaired. Repair solution 1, Repair solution 2, and Repair solution 3 are solutions in which a row address of failed cell d is repaired, and Repair solution 5 is a solution in which a column address of failed cell d is repaired. Similarly, Repair solution 1, Repair solution 2, and Repair solution 3 are solutions for failed cells g and h in which a row address of failed cell a and a row address of failed cell d are repaired.
According to the repair method of the prior art shown in FIG. 1, when failed cells (indicated by an "x") are present in a memory cell as shown in FIG. 5, the number of repair spare rows is five (their addresses being R1-R5) and the number of repair spare columns is two (their addresses being C1 and C2).
Because there are only four spare rows per block, one row of the five is repaired using the spare row A portion of the neighboring block, as shown in FIG. 6. For example, when the failed cells of addresses R2.sub.-- R5 are repaired by spare rows of the same block (block 1), and the failed cells of address R1 are repaired using the spare row of the neighboring block, the failed cell of address R1 indicated by a circle cannot be repaired. This is because the spare row B portion (referred to as the "limbo" portion) can only be used to repair a failed cell of a spare column of its own block.
As a consequence, repair of a device cannot be achieved despite replacement by the user of a defective site with a spare cell (spare row) based on a repair solution found by the repair method of the prior art.